What is the potential of avirulent influenza viruses to complement a cleavable hemagglutinin and generate virulent strains?

A large pool of avirulent influenza viruses are maintained in the wild ducks and shorebirds of the world, but we know little about their potential to become virulent. It is well established that the hemagglutinin (HA) is pivitol in determining virulence and that a constellation of other genes is also necessary (R. Rott, M. Orlich, and C. Scholtissek, 1976, J. Virol. 19, 54-60). The question we are asking here is the ability of avirulent influenza viruses to provide the gene constellation that will complement the HA from a highly virulent virus and for the reassortant to be virulent. Reassortant influenza viruses were prepared between ultraviolet treated A/Chicken/Pennsylvania/1370/83 (H5N2) [Ck/Penn] and influenza viruses from natural reservoirs. These viruses included examples of the predominant subtypes in wild ducks, shorebirds, and domestic poultry. Attention was given to the influenza viruses from live poultry markets, for it is possible that these establishments may be important in mixing of influenza genes from different species and the possible transmission to domestic and mammalian species. The reassortants were genotyped by partial sequencing of each gene and were tested for virulence in chickens. Each of the reassortants contained the hemagglutinin and matrix (M) genes from Ck/Penn and a majority of genes from the viruses from natural reservoirs indicating a preferential association between the HA and M genes. The reassortants containing multiple genes from wild ducks and a cleavable HA were avirulent indicating that the gene pool in ducks may not have a high potential to provide genes that are potentially virulent. In contrast, a disproportionate number of viruses from shorebirds and all avirulent H5N2 influenza viruses from city markets provided a gene constellation that in association with cleavable H5 HA were highly virulent in chickens. An evolutionary tree based on oligonucleotide mapping established that the H5N2 influenza viruses from birds in city markets are closely related.